The present invention relates generally to electronic controls for operating an internal combustion engine, and more particularly, to a method and apparatus for detecting a misfiring within a cylinder during operation of the engine.
Most automobiles employ a catalytic converter to reduce the amount of pollutants in engine exhaust. Engine misfire occurs when the combustion process is incomplete or does not occur. When a cylinder misfires, unburned fuel escapes into the exhaust system.
State and federal agencies have identified engine misfire as a contributor to vehicle emissions and a cause of poor catalyst performance. They have instituted regulations that require misfire detection systems be implemented in automobile on-board diagnostics to alert drivers (technically through an indicator on the instrument panel) when a misfire measurement rate exceeds a threshold mandated by such agencies.
The aim of the regulations is to determine an accurate measurement of the misfire rate and to alert the driver as soon as possible when a misfire rate exceeds thresholds enumerated in the regulations. There are three important parameters to consider in order to carry out regulations established by the agencies. They are as follows: 1) threshold rate Rth (the rate of misfire determined to cause emissions that are too high; 2) time constant (the length of a time xe2x80x9cwindowxe2x80x9d over which measurements of the average misfire rate is taken); 3) false alarm rate Rfo (frequency of occurrence of normal firings which are incorrectly classified as misfirings) and 4) the rate at which real misfires are incorrectly classified as normal.
In one known misfire detection system, a fixed window is used. The window is the time during which misfire is detected. In the known system, the window is fixed relative to the crank angle. Having a fixed regular occurring window is suitable for many engine applications. However, some engines have firing events at irregular intervals because of the inherent geometry of the crankshaft and engine design. For example, in some xe2x80x9cVxe2x80x9d block engine designs, one engine bank may have the misfire window centered around the potential misfire event while the other bank may be up to 30 degrees off center from the optimum misfire detection time.
Various filters are also used to process the misfire signal. Commonly, a 0.01 order filter and a 1.0 order filter are used. The 0.01 order filter is used at higher engine speeds and the 1.0 order filter is used at lower engine speeds. Each of these filters, however, has a different phase delay. The different phase delays also account for some shift of the misfire signal with respect to the misfire window. Other engine conditions such as ignition timing and load may also contribute to a misalignment of the misfire signal with respect to the misfire sampling window.
It is important to align the misfire signal with the misfire window so that a proper measurement may be made. Fixed windows, which are calibrated at a fixed width and position relative to the crankshaft angle, occur at evenly spaced intervals. The misfire signal is a measurement of the minimum acceleration value that occurs within the window. That is, the misfire signal is the minimum acceleration value that occurs when the misfire window is xe2x80x9chigh.xe2x80x9d
For engine diagnostic applications, it is desirable to track the misfire rate separately for each engine cylinder. Therefore, it would be desirable to provide a method for accurately providing a misfire detection window that is capable of adjusting to accurately align with the misfire signal.
In one aspect of the invention, an engine misfire detector includes a reference window generator that generates a misfire window reference signal having a misfire sampling window. A misfire window calculator is coupled to a condition variable input and the reference window generator. The condition varying input may be a filter input or an input from various engine sensors. The misfire window calculator calculates an offset for adjusting the sampling window in response to the condition variable input.
In a further aspect of the invention, a method for determining misfire of an engine generally comprises the steps of obtaining a reference misfire window signal and adjusting the reference misfire detection window in response to a condition varying input to obtain an adjusted window signal.
One advantage of the invention is that both the window lengths and the relative position of the window may be adjusted based upon the filter type in use, the load, spark, age, or other conditions that may affect the alignment of the misfire signal with respect to the misfire detection window.